1. Field of the Invention
This invention relates to a pyroelectric infrared array sensor which senses infrared rays from a human body to give an alarm signal.
2. Description of the Related Art
A pyroelectric infrared array sensor utilizes a sensing element comprising a material such as PZT (lead zirconate titanate) having a pyroelectric effect. The term "pyroelectric effect" as used herein is intended to mean the characteristic that, when infrared rays are applied to the sensing element, the surface temperature of the latter is changed, as a result of which the charges therein are no longer in the state of neutralization; that is, the element becomes electrically unbalanced, thus producing charges. The charges thus produced can be converted to a voltage by an impedance.
An equivalent circuit of the pyroelectric infrared array sensor is, for instance, as shown in FIG. 1. A sensing section 1 is formed on a pyroelectric element, and is connected in parallel to a high-resistance chip 2. One of the terminals of the high-resistance chip 2 is connected to the gate terminal of an FET (field-effect transistor), and the other terminal is grounded. When a positive voltage is applied to a drain terminal pin 4 connected to the drain terminal of the FET 3, charges produced in response to the application of infrared rays to the sensing section 1 can be obtained as a voltage output at a source terminal pin 5 connected to the source terminal of the FET 3.
This characteristic is utilized to provide a pyroelectric infrared linear array sensor in which a plurality of sensing sections are arranged in a line, or a pyroelectric infrared two-dimensional array sensor in which a plurality of sensing sections are arranged two-dimensionally. Those sensors are used for detecting the direction of movement or position of a person or the like.
FIG. 2 is an exploded perspective view of the pyroelectric infrared array sensor. Terminal pins 6 are embedded in a metal header 8 in such a manner that they penetrate the header 8 through insulating cylindrical pieces 7. A grounding pin (not shown) is formed as follows: With conductive paste applied to the insulating cylindrical piece 7 of a selected one of the terminal pins 6, the terminal pin 6 is shorted to the metal header 8, to form the desired grounding pin.
The terminal pins 6 protruded from the upper surface of the metal header 8 are inserted into holes 10 formed in a substrate 9, respectively, and fixedly connected to lands (not shown) which are connected to a circuit (not shown) formed around the holes 10.
A pyroelectric element 11 is mounted on the upper surface of the substrate 9, and high-resistance chips 2 and FETs 3 are mounted on the upper and lower surfaces of the substrate 9. The number of the high-resistance chips 2 and that of the FETs 3 are equal to the number of the sensing sections.
A casing 12 has an opening 13 which confronts the pyroelectric element 11. The opening 13 is closed with an infrared filter 14.
The metal header 8 is electrically welded to the casing 12, so that the header 8 is electrically connected to the casing 12.
The pyroelectric element 11 is polarized in advance so that its one side is positive, and the other side is negative. The light receiving surface of the pyroelectric element 11 is confronted with the infrared filter 14 which is perpendicular to the axis of polarization. As shown in FIG. 3, a plurality of electrodes 15A are provided on the light receiving surface of the pyroelectric element 11, and a plurality of electrodes 15B are provided on the opposite surface of the element 11 in such a manner that the electrodes 15B are confronted through the element 11 with the electrodes 15A, respectively, thus providing a plurality of sensing sections each including a pair of electrodes 15A and 15B. The electrodes 15A of the sensing sections are connected to one another with connecting conductors 15C. In addition, lead conductors 15D are formed on the element 11; that is, the electrodes 15A thus connected to each other are connected to the circuit on the substrate 9 through the lead conductors 15D. The electrodes 15A and 15B, the connecting conductors 15C, and the lead conductors 15D are formed by vapor deposition of NiCr, Ag, Ag--Cu or the like. In the case where the electrodes 15A and 15B are of Ag or Ag--Cu, a black film is formed on the surface of each of the electrodes 15A, which absorbs heat with high efficiency.
In order to prevent heat generated at the sensing sections of the pyroelectric element 11 from flowing to the substrate 9, the pyroelectric element 11 is held spaced from the substrate 9; that is, it is secured by the electrodes 15B to solder bumps 16 formed on the circuit on the substrate 9 by using conductive paste 17. Each of the electrodes 15B is connected through the circuit on the substrate 9 to one terminal of the respective high-resistance chip 2 and to the gate terminal of the respective FET 3. The electrodes 15A, which are connected to one another with the connecting conductors 15C, are fixedly connected through the lead conductors 15D to conductive parts such as the solder bumps on the circuit of the substrate 9 by using the conductive paste 17A, and connected to the grounding pin and to the remaining terminals of the high-resistance chips 2. The drain terminals and source terminals of the FETs 3 are connected to predetermined ones of the terminal pins 6.
Charges produced by the plurality of sensing sections of the pyroelectric element 11 are obtained as voltage by a plurality of impedance conversion circuits made up of the FETs 3. By comparing the different outputs of the plurality of sensing sections with one another, the direction of movement or position of a person or the like can be detected.
In the case of a pyroelectric infrared two-dimensional array sensor which includes sixteen (4.times.4) sensing sections 18 through 33 as shown in FIGS. 4(a) and 4(b), the sensing sections 18, 21, 30 and 33 at the four corners are each adjacent to two sides of the rectangular pyroelectric element 11, and therefore they are high in thermal resistance. Hence, it is difficult for the heat generated at those sensing sections by infrared rays to diffuse. The sensing sections 19, 20, 22, 25, 26, 29, 31 and 32 are provided along respective sides of the pyroelectric element 11. Therefore, the heat generated at the sensing sections 19, 20, 22, 25, 26, 29, 31 and 32 is less diffused than the heat generated in the sensing sections 18, 21, 30 and 33. Hence, the sensing sections 18, 21, 30 and 33 are highest in sensitivity, and the sensing sections 19, 20, 22, 25, 26, 29, 31 and 32 are next.
However, the pyroelectric infrared two-dimensional array sensor is not able to deal with this variation in sensitivity. Hence, in application of the sensor, the amplification factors of the amplifier circuits for all the sensing sections are adjusted; in other words, the adjustment is carried out for every sensing section, which takes a lot of time and labor, and increases the manufacturing cost of the sensor.
In a pyroelectric infrared linear array sensor as shown in FIGS. 5(a) and 5(b) in which a plurality of sensing sections are arranged one-dimensionally, as in the case of the pyroelectric infrared two-dimensional array sensor, the sensing sections at both ends of the pyroelectric element 11 are highest in sensitivity, and the remaining sensing sections are lower in sensitivity towards the middle of the pyroelectric element 11.
Hence, the sensing sections at both ends of the pyroelectric element 11 are employed as dummy sensing sections, or they are positioned away from both ends of the element 11, which results in an increase in size of the element 11. Hence, it is impossible to miniaturize the pyroelectric infrared linear array sensor.
Further, the above-described conventional pyroelectric infrared array sensor suffers from the following difficulties: In the conventional sensor, referring again to FIG. 3, when temperature is changed, the conductive paste 17A is expanded or contracted. In this case, the conductive paste 17A is provided only at one position as was described above. Hence, in response to the expansion or contraction of the conductive paste 17A, a great stress is applied to the pyroelectric element 11 in the direction of the arrow F, with the bump 16A closest to the conductive paste 17A as a fulcrum as shown in FIG. 3. As a result, the solder bumps 16 of the sensing sections elements may be cracked at the connecting points or may come off, thus impairing the reliability of the infrared array sensor.
In addition, depending on how the solder bumps are cracked at the connecting points or come off, the electrical conduction between the pyroelectric element 11 and the substrate 9 is deteriorated, which increases noise and lowers the sensitivity of the sensor.